1. Field of the Invention
This invention relates to photographic elements and more particularly to photographic film supports for improved photographic elements.
2. Description of Related Art
Plasticized cellulose triacetate has found widespread use as a photographic support. These supports are prepared by casting a cellulose triacetate dope on a wheel or band and allowing the solvent to evaporate causing the sheet to cure to a point where it can be stripped from the casting surface for subsequent processing. In a refinement of this process a second layer of dope is applied to a previously cast triacetate sheet and subsequently dried to build a two-layer structure. The principal advantage is higher speeds for multi-layer casting relative to single-layer casting.
As a practical matter, photographic support must be recyclable. To simply discard normal manufacturing waste would be cost-prohibitive. A typical recovery cycle for flaked cellulose triacetate waste removes any emulsions or backing layers present in a sequence of wash and rinse cycles and then redissolves them in solvents to make a dope for casting. This dope must produce clear and homogenous support.
To overcome the problem of accumulation of static charges, it is conventional practice in the preparation of photographic elements to provide an antistatic layer. A wide variety of antistatic layers are known for use in photographic elements. One particularly useful antistatic agent described in U.S. Pat. No. 4,203,769 is vanadium pentoxide. This antistatic layer is commonly overcoated with a barrier layer in order to protect the antistat from degradation by photographic developing solutions. These overcoats can either be an outer backing layer of the film or can be in turn overcoated with additional functional layers such as the photographic emulsion layers.
These barrier layers or overcoats serve the function of preventing degradation of the antistat layer satisfactorily; however, they increase the complexity of the product and its manufacturing process, thus increasing the costs. Furthermore, the barrier properties of these layers for water in developer solutions imposes specific performance requirements which may interfere with other aspects of support performance. For example, hydrophobic layers can interfere with adequate draining of the film support during processing, thus leaving water marks on the developed images. Further, should it be desired to place the antistat layer on the emulsion side of the film support, it is generally necessary to include an adhesion promoting layer because emulsion layers have poor adhesion to hydrophobic layers.
In general, antistats used in photographic products are positioned at or near the surface to facilitate the dissipation of charge accumulated by transport through the manufacturing operations. While effective for this purpose, this location of the antistat layers renders its conductivity to be susceptible to being reduced or entirely eliminated by photographic developing solutions. An example of an antistat removed intentionally is the conductive carbon coating, also used as an antihalation layer, which is subsequently removed to permit projection. An example of an antistat layer, the conductivity of which is inadvertently reduced, is the ionic conductor poly(benzyltrimethylammonium chloride-coethyleneglycoldimethacrylat) which is used in various negative and positive image forming elements. Thus, there is a need for a permanent, process surviving antistat for photographic elements in order to reduce dirt accumulation on developed film negatives.